We have continued to explore the hormonal and nutritional regulation of metabolism along the following lines: (a) Enhancement of flavin synthesis in newborn rats by maternal riboflavin deficiency. This study determined that maternal riboflavin deficiency enhances the rates of incorporation of radioactive riboflavin into radioactive flavin mononucleotide (FMN) and radioactive flavin adenine dinucleotide (FAD) as well as into acid non-extractable flavins in liver and in brain of newborn animals. (b) Thyroid hormone control of covalently-bound flavins in liver and brain. In cerebrum of adult male rats, triiodothyronine (T3) and thyroxine (T4) increased the rate of incorporation of 14C-riboflavin into covalently-bound flavins. These data suggest that thyroid hormones may exert selective effects upon brains of adult animals. (c) Patterns of recovery of disordered taste sensations in hypothyroid patients. Elevated taste thresholds for saline and sucrose in hypothyroid patients were corrected more quickly and more completely by treatment with thyroid hormones than were the thresholds for HCl and quinine. Further work in experimental animals has documented abnormal taste preferences of hypothyroid rats and their correction by thyroxine. (d) Regulation of L-triiodothyronine aminotransferase activity by T3 in normal and neoplastic tissues. These studies suggest that T3 may regulate its own rate of transamination in liver both of normal and of tumor-bearing rats, but not in tumor tissue.